PROJECT SUMMARY Cholestatic liver diseases are highly prevalent causes of progressive liver disease in the United States with a significant morbidity and mortality. Unfortunately, current medical therapies frequently do not prevent disease progression and are not curative. Over the past decade, the Unfolded Protein Response (UPR), an adaptive cellular response to Endoplasmic Reticulum (ER) stress, has been implicated in the pathogenesis of many liver diseases. However, the role of the UPR in hepatic bile acid toxicity and cholestatic liver injury remains poorly understood. The Inositol-Requiring Enzyme 1?/X-box binding protein 1 (IRE1?/Xbp1) pathway is a highly evolutionarily conserved signaling pathway of the UPR that is both protective to the liver and is important in the regulation of lipid metabolism. The central hypothesis of this proposal is that hepatic IRE1?/XBP1s signaling regulates bile acid metabolism, and that bile acid signaling further regulates the IRE1?/XBP1s pathway. We show preliminary data demonstrating that the hepatic IRE1?/Xbp1 pathway is activated by cholestasis, is an important protective response to reduce cholestatic liver injury and regulates bile acid metabolism. Therefore, we will determine the role of the hepatic IRE1?/XBP1 pathway in the regulation of cholestatic liver injury (Specific Aim 1A) and bile acid synthesis (Specific Aim 1B). Hepatic FXR/SHP signaling is an essential bile acid-responsive pathway that regulates many genes and physiologic processes involved in bile acid metabolism. In preliminary studies, we demonstrated that the IRE1?/XBP1s pathway is regulated by the FXR/SHP signaling pathway. Thus, we will define the regulatory mechanisms of FXR/SHP signaling on the hepatic IRE1?/Xbp1s pathway (Specific Aim 2). Finally, FGF19 is an ileal hormone produced in response to bile acids that regulates bile acid synthesis and other hepatic processes. Therefore, we will characterize the regulation of the IRE1?/Xbp1 pathway by FGF19 signaling (Specific Aim 3). Our long- term goal is to further develop a line of research characterizing the mechanisms by which the IRE1?/XBP1s and other UPR signaling pathways reduce liver injury during cholestasis and other forms of liver disease. This proposal utilizes state-of-the-art mouse genetics, molecular biology, proteomics and physiologic techniques to further determine the protective role of the IRE1?/XBP1, FXR/SHP and FGF15/19 signaling pathways in bile acid injury and cholestasis. These investigations may help identify novel regulatory mechanisms of bile acid metabolism and IRE1?/XBP1 signaling that can be used to target new therapies for the treatment of cholestatic liver disease and other hepatic disorders.